Synthesis of polycarbosilane/siloxane hybrid polymers and their pyrolytic conversion to silicon oxycarbide ceramics

Polycarbosilane/siloxane hybrid polymers were synthesized by sol-gel processing of the cyclic organosilicon compound 1,1,3,3-tetraethoxy-1,3-disilacyclobutane (CBS) and the linear polymer poly(diethoxysilylenemethylene) (L-EPCS) derived from CBS by ring-opening polymerization. The structure of these polymers and their pyrolysis to silicon oxycarbide ceramics were investigated by elemental analysis, thermogravimetric analysis, FT-IR, and solid-state NMR spectroscopy. The results obtained indicate that the gel, obtained by hydrolysis/condensation of CBS with acid catalyst, underwent a Si-OH-induced, ring-opening reaction before the organic-to-inorganic conversion took place during pyrolysis, leading to the formation of intermolecular Si-O-Si(Me) linkages. In the case of the linear polycarbosilane (L-EPCS), sol-gel processing under acid catalysis leads to a gel of the type [SiOCH2](x)[Si(OR)O0.5CH2](y) (R = H or Et). Si-29 SSNMR spectra show that this gel, which has a nearly pure SiC2O2 microenvironment at; the outset, converts into a silicon oxycarbide that contains a statistical distribution of the five possible SiC4-xOx environments between 600 and 1000 degrees C. This rearrangement of the Si environments was attributed to redistribution reactions involving the exchange of Si-O and Si-C bonds during the latter stages of the pyrolysis, Likely facilitated by an analogous Si-OH-induced attack on the Si-CH2-Si linkages. Both SSNMR and elemental analysis indicated that bridging CH2 groups have the effect of increasing the carbidic carbon content and decreasing the free carbon content in the resulting silicon oxycarbide ceramics relative to polymers with pendent Si-CH3 groups.